1. Technical Field
This invention relates to fiber/metal laminates, and more particularly to an aramid/aluminum laminate having greatly improved fatigue characteristics and being particularly suited to use as an aircraft fuselage material.
2. Background Information
Two major concerns in the manufacture of aircraft structural components, particularly the fuselage, are fatigue strength and weight. Sheet aluminum is the state-of-the-art fuselage sidewall material because of its light weight and relatively easy workability. However, one of the greatest aging aircraft problems facing the industry today is the weakening of the pressure cabin due to multiple-site fatigue damage, which is caused by repeated pressurization and de-pressurization of the cabin during takeoffs and landings. Aluminum is sensitive to fatigue stress, and therefore a number of alternatives have been considered in an attempt to improve fatigue life while retaining aluminum's lightweight characteristics and workability.
The most successful and cost efficient alternatives have been composite materials, particularly fiber/metal laminates. The earliest available fiber/metal laminates were made of aramid and aluminum (such as the ARALL.RTM. laminates manufactured by a division of the Aluminum Corporation of America (ALCOA), located in Parnassus, Pa. These laminates were developed for high fatigue crack growth resistance. However, tests have shown that unstretched aramid/aluminum laminates can experience an adverse fiber failure phenomenon, subject to certain loading conditions.
A solution to the problem of adverse fiber failure can be obtained by "post-stretching" the aramid/aluminum laminate. By this procedure, the laminate is placed in tension along a post-stretch axis which lies parallel to the fiber orientation, and is thereby stretched. Force is applied until the metallic layers yield plastically and the entire laminate is permanently elongated a predetermined amount. Since the fibers remain elastic throughout the process, they retain a certain degree of residual tensile stress after the stretching operation is complete. The effect is essentially to pre-stress the fibers. The tensile stress in the fibers induces a reactive compressive stress in the metallic layers. The result is a laminate with increased tensile yield strength, improved fatigue performance (prestressed fibers hold fatigue cracks in the metal layers more firmly shut, impeding crack growth), and improved resistance to fiber failure (due to the residual tension, the fibers are less vulnerable to repeated microbuckling).
This post-stretching solution has been applied successfully to unidirectional laminates, i.e. laminates having fibers oriented in only one direction. The post-stretch axis is parallel to the fiber orientation, as discussed above, and the above-mentioned properties are generally realized. However, there are many applications, such as an aircraft fuselage sidewall, which require strength properties provided only by laminates which are reinforced with bi-directionally oriented fibers. Such bi-directionally reinforced laminates, typically having a fiber orientation of 0.degree. and 90.degree., usually are unstretched, because stretching along one fiber direction (for example, the 0.degree. direction) will cause the metal to contract in the transverse direction, thus inducing undesirable compressive stresses in the transverse (90.degree.) fiber plies. No suitable process has been developed to date for bi-directionally post-stretching fiber/metal laminates. Unstretched bi-directionally reinforced aramid/aluminum laminates have inadequate fiber failure resistance, and therefore these laminates have not heretofore been suitable for use in such fatigue-critical applications.
Another potential solution to the fatigue problem discussed above is to use fiberglass reinforced aluminum laminates, which, when reinforced by bi-directionally oriented glass fibers have shown good fiber failure resistance even without post-stretch. However, the glass fibers are much heavier than aramid fibers and have a lower modulus, and thus are not as suited to weight-critical aircraft applications.
What is needed, therefore, is a fiber-failure resistant aramid reinforced aluminum laminate having bi-directionally oriented fibers.